1. Field of the Invention
The present invention relates to digital data transmission using modems and more particularly, to adjustment in transmitting speed thereof.
2. Description of the Prior Art
A telephone line is used as a transmitting line of communication between computers. Since the telephone line is arranged for transmitting human voice, only signals having audio frequency, i.e., 300 Hz through 3000 Hz can be transmitted. Since digital signals from a computer 2 (see FIG. 10) have a pulse form, digital signals can not be transmitted through the telephone line.
A modem modulates the digital signals so that they have audio frequency and then the modulated signals are sent through the telephone line to an opposite modem connecting to an opposite computer. In this modulation, distinguishing between a logic "1" level and a logic "0.infin. level is effected by using phase difference. The opposite modem demodulates the modulated digital signals and the demodulated digital signals are sent to the opposite computer 8. Thus, modems are used to effect modulation and demodulation.
Under requirements from the Electronic Industries Association (EIA), data signal speed through the telephone line is fixed at a multiple of 300 bits per second (bps). Specifically, these are 300, 1200, 2400, 4800, 9600, 19200, 38400 bps.
Data signal speed depends on the frequency of the "transmitting speed clock". "Transmitting speed clock" is determined by how many pieces the first reference clock can be divided into. The relation between the first reference clock of 5.296 MHz and various division numbers is shown in FIG. 1.
Similarly, transmitting speed in a computer is determined by how many pieces a second reference clock of the computer is divided into. When, for example, the computer has an operating frequency of 9.8304 MHz, the second reference clock is 153600 Hz. Therefore, the relation between the second reference clock of 153600 Hz and various division numbers is shown in FIG. 2. Referring to FIG. 2, data signal speeds of 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 76800 and 153600 bps can be obtained using the division numbers of FIG. 2.
However, the described-above prior art includes the following problem.
Personal computers on the market have an operating frequency of 7.9872 MHz, not the operating frequency of 9.8304 MHz. FIG. 3 shows the relation between various division numbers and the desired data signal speeds of 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 76800 and 153600 bps when a personal computer has the operating frequency of 7.9872 MHz. Apparently from FIG. 3, data transmission can not be completed at each of data signal speeds of 19200, 38400, 76800 or 153600 bps when the personal computer has the operating frequency of 7.9872 MHz (or the second reference clock is 124800 Hz).